BRUSSELS – In a letter to senior Trump administration officials, European foreign and finance leaders this week tacitly acknowledged that their efforts to preserve the West’s nuclear deal with Iran were failing.

In the letter, sent on Monday to Treasury Secretary Steven Mnuchin and Secretary of State Mike Pompeo, the European leaders cited “security interests” in requesting that companies in Europe be granted an exemption from U.S. sanctions that would be imposed as a result of President Donald Trump’s decision to withdraw from the pact.

“In their current state, U.S. secondary sanctions could prevent the European Union from continuing meaningful sanctions relief to Iran,” said the letter, signed by the finance and foreign ministers of Britain, France and Germany, all signatories to the 2015 accord with Tehran, and by Federica Mogherini, the E.U.’s foreign-policy chief, who oversaw the negotiations.

Without that sanctions relief, Iran has threatened to pull out of the deal. That “would further unsettle a region where additional conflicts would be disastrous,” the ministers argue, asserting that the deal was “the best means through which we can prevent a nuclear-armed Iran,” and that there appeared to be “no credible alternatives at this time.”

The letter was first reported by the Wall Street Journal.

The plea is considered unlikely to produce the relief the Europeans want, since the Trump administration’s stated intention is to pressure Tehran into agreeing to an entirely new set of negotiations that would encompass its ballistic missile program and its support for hard-line regimes and militias throughout the Middle East.

The Europeans have been working with the State Department for some time to try to persuade Trump to remain in the nuclear agreement. They proposed discussing parallel new sanctions on Iran for its missile program, its support of the Syrian government and its backing of the Houthi rebels in Yemen and other groups. But the Europeans were unable to convince U.S. officials that they could fix the so-called sunset provisions in the original deal, which allow Iran to resume various nuclear activities after a period of years.

Tehran has always denied that it ever intended to build a nuclear weapon, even though Western intelligence agencies said Iran tried to do just that before abandoning the effort in 2003. While Iran continued to work on ballistic missiles, there has been no evidence found of a nuclear weapons program since that date.

On Tuesday, however, Iran announced that it was preparing its nuclear facilities to resume large-scale uranium enrichment and had built a factory for constructing advanced centrifuges should Europe fail to preserve the deal.

Iran’s leadership is divided, but some would prefer to stay in the deal even without the Americans, to avoid possible military strikes from Israel and from the United States and to try to upgrade the economy. But that prospect seems unlikely if European companies are denied a waiver from the “secondary” sanctions.

Iran negotiated the 2015 deal in order to get relief from those crippling secondary sanctions, which Washington is now reimposing and which would deny access to financial systems in the U.S. to any company doing business with Iran.